Stem cell therapy is rapidly advancing into the clinic, but there remains a critical need for effective experimental models for in vivo analyses of human stem cells to evaluate their efficacy and safety. Although rodent models have provided fundamental insights into disease mechanisms, mice and humans differ in terms of cell composition, function, and gene expression. We propose to address this species-specific issue using novel humanized mouse models that meet urgent needs of multiple institutes at NIH. Our novel models of humanized mice based on our creation of NOD-scid IL2rgnull (NSG) and NOD-Raglnull IL2rgnull (NRG) strains will support the in vivo study of human stem cell biology and permit evaluation of the efficacy of human stem cells for the treatment of diseases. Our preliminary data describes development of novel humanized mouse models that support engraftment with functional human hematopoietic, lymphoid, and hepatic stem cells and that we are developing models for the study of human neuronal stem cells for the treatment of amyotrophic lateral sclerosis and muscle stem cells for the treatment of muscular dystrophy. In all of these models, the mechanisms underlying the ability of human mesenchymal stem cells to enhance engraftment of tissue-specific human stem cell populations and increase their therapeutic efficacy can be investigated. In this multi-PI, multi-disciplinary team proposal, we propose three Aims that will: 1) Generate new models of immunodeficient mice for the functional in vivo evaluation of human stem cells; 2) Validate the models by determining the ability of human stem cells to generate functional differentiated human cells and tissues in vivo that ameliorate disease, and; 3) Leverage the world-wide distribution resources of The Jackson Laboratory to make these new humanized mouse models rapidly available to the scientific community. Our proposal takes advantage of powerful new technologies for creating new models of humanized mice, and builds on our >20 year track record for generating, validating, and sharing of novel models of humanized mice. These models developed and validated by our multi-disciplinary team will be rapidly distributed to the scientific community, uniquely positionig us to achieve the goals of this RFA.